In the production of sulphate pulp, soda pulp and sulphite pulp with an alkali metal as a base, normally sodium, it is possible to recover the inorganic pulping chemicals in the spent liquor leaving the digester. It is vital both to economy and environment to recover these pulping chemicals to the largest possible extent. This is achieved in the pulping chemical recovery system, which essentially transfers the used inorganic pulping chemicals into a chemical state, where they can be re-used for cooking.
An essential part of the recovery system is the recovery boiler, where the spent liquor is burned. Normally, make-up chemicals are added to the spent liquor before the recovery boiler to make up for the chemicals lost during cooking and recovery. The spent liquor is sprayed into the lower part of the boiler, previously at a relatively low temperature to remove free water. Modern recovery boilers operate at a high temperature to reduce the content of sulphur in the flow gases leaving the boiler. Higher up in the boiler, gases and vapours of light hydrocarbons and decomposition products are volatilized. This is known as pyrolysis. Then, the pyrolysis products are burned after mixing with air or oxygen. The solid carbon-based residue which remains after complete pyrolysis of the organics is then heterogeneously burned. The solid particles formed are collected as a dust in precipitators at the top of the recovery boiler, to reduce the release of solid material to the surrounding atmosphere.
A substantial and increasing problem with the pulping chemical recovery system, is the presence of chloride and potassium in the spent liquor entering the recovery boiler. These elements tend to reduce the capacity of the recovery boiler to produce useful chemicals. Thus, chloride and potassium increase the stickiness of carryover deposits and dust particles to the recovery boiler tubes, which accelerate fouling and plugging in the upper part of the recovery boiler. Chloride also tend to increase the corrosion rate of superheater tubes.
Chloride and potassium are concentrated in the dust formed during the combustion of spent liquor in the recovery boiler. The dust is collected in dry-bottom or wet-bottom electrostatic precipitators. The dust mainly consists of sodium and potassium salts, where sulphate, carbonate and chloride are the dominant anions. The amount of dust corresponds to about 5 to about 15% by weight of the sodium entering the recovery boiler, which corresponds to about 50 to about 150 kg dust per ton pulp, if the dust is calculated as sodium sulphate.
Today, normally all of the precipitator dust collected and withdrawn from the recovery boiler is recycled to the flow of spent liquor to be burned in the boiler. When the concentration of chloride or potassium is too high, a portion of the precipitator dust is withdrawn from the system and discharged or deposited.
The content of chloride in the spent liquor can be very high for coastal mills, if the raw material consists of logs floated in seawater. The content is moderate in mills using caustic make-up contaminated with sodium chloride or in mills that at least partially recover spent bleach liquids from stages using chlorine-containing bleaching agents. As the environmental legislation becomes more stringent regarding pulp mill discharges to air and water, the degree of system closure increases. This means that even a small input of chloride becomes a severe problem, unless the content can be controlled by purging the system in some environmentally acceptable way.
A further problem in the chemical recovery system, in the treatment of spent liquors and recirculation of the purified process liquids, is the content of metal ions. In the treatment of the spent liquors, especially when using electrochemical methods, the metals are harmful. Metal ions such as calcium (Ca) and magnesium (Mg) may precipitate on the membranes and cause damage on the membranes. Ca and Mg may also form sparingly soluble salts which are clogging the compartments of the cell, thus leading to an interruption in the production due to restoration of the cells.
Several methods have been proposed to overcome the problem with chloride and potassium build-up in pulping chemical recovery systems. One example is evaporation of cooking liquid to recrystallize sodium chloride and potassium chloride. Also known is leaching of precipitator dust and discarding the leach liquid rich in chloride.
According to Tran et al., Pulp Paper Canada 91(5): T185-T190 (1990), the easiest and most effective way to control chloride, as well as potassium, in the chemical recovery cycle today, is by directly discarding the precipitator dust. Therefore, still the most commonly used method is removal of part of the precipitator dust from the system, and subsequent deposition on land or discharge to water. However, this will not only be environmentally unacceptable, but also result in a loss of valuable cooking chemicals.
U.S. Pat. No. 5,352,332 discloses a process for recycling bleach plant filtrate. Precipitator dust is collected and treated by leaching with water or by evaporation crystallization from a water solution. The thus formed salt solution is discharged to sewer or recovered for its chlorine value.
WO-A1-9404747 discloses a process, in which the content of chloride in a recovery system for pulping chemicals can be reduced. The process comprises collecting precipitator dust, dissolving the dust in water to produce an aqueous solution of precipitator dust, whereupon said aqueous solution is electrolysed in a cell for production of chlorine or hydrochloric acid in the anolyte.
JP-A-55022051 discloses a process for reduction of chloride where precipitator dust is washed with a Glauber's salt solution, whereafter a part of the washing solution is treated by electrodialysis to remove chloride.
CA 1059271 discloses a process for reduction of chloride in a pulp mill recovery system. Precipitator dust is leached with hot water at a temperature of 60-100.degree. C. Chloride is precipitated from the leached solution by cooling crystallization. Solid sulphate is recycled to the black liquor. Acid (sulphuric acid) is added in the leaching to lower the pH in order to precipitate sulphate.